1. Field of the Invention
The present invention relates to a synchronous memory device, and in particular to an improved synchronous memory device having an asynchronous precharge function which can input an interval between commands regardless of a clock period. It accomplishes this by generating a precharge control signal asynchronous with respect to a clock signal by using a clock enable signal or chip selector bar signal which is not synchronous with the clock signal.
2. Description of the Background Art
A conventional synchronous DRAM will now be explained with reference to FIGS. 1 through 5. FIG. 1 (Prior Art) is a block diagram illustrating a conventional synchronous memory device. The conventional synchronous DRAM includes a command buffer 20 receiving external commands such as a clock signal CLK, a chip selector signal /CS, a ras bar signal /RAS, a cas bar signal /CAS and a write bar signal /WE; an address buffer 22 receiving external addresses A0-Ai; and a data IN/OUT buffer 24 receiving and outputting data signals DQ0-DQj.
The conventional synchronous DRAM also includes a command decoder 26 receiving the output signals /CSI, /RASI, /CASI, /WEI from the command buffer 20, and generating a precharge control signal PREI for controlling the internal operation, a write signal W, an active signal ACT, a read signal R and a mode selection signal MS. A mode register 28 receives the output signal from the address buffer 22 and the mode selection signal MS from the command decoder 26, and controls an operation mode. A test mode detector 34 receives the address signal A0 and the output signal from the mode register 28, and generates a test mode detection signal TEST. A precharge control signal generator 60 comprising a TPRE signal generator 36 receives the write signal W from the command decoder 26, the write enable signal /WEI from the command buffer 20, and the detection signal TEST from the test mode detector 34, and generates a test mode precharge control signal TPRE. An OR gate 38 ORs the precharge control signal PREI from the command decoder 26 and the TPRE signal from the TPRE signal generator 36, and generates a precharge control signal PRE.
In addition, the conventional synchronous DRAM includes a column address latch unit 30 latching the column address signal from the address buffer 22, and generating a column address latch signal CA. A column decoder 40 decodes the column address latch signal CA from the column address latch unit 30, and selects a bit line of a memory cell array unit 50. A row address latch unit 32 receives the address signal from the address buffer 22, and generates a row address latch signal RA. A row decoder 42 decodes the row address latch signal RA from the row address latch unit 32, and selects a word line of the memory cell array unit 50.
The conventional synchronous DRAM includes a precharge/equalize unit 44 precharging and equalizing a bit line potential of the memory cell array unit 50 into half a potential xc2xdVcc according to the precharge control signal PRE from the OR gate 38, when data are not written/read to/from the memory cell array 50. A sense amplifier unit 46 precharges and equalizes the bit line potential into half a potential xc2xdVcc according to the precharge control signal PREI, when data are not written/read to/from the memory cell array 50, and amplifies data of the bit line, when data are written/read to/from the memory cell array 50. I/O unit 48 connects the data inputted/outputted through the data IN/OUT buffer 24 to the sense amplifier unit 46.
FIG. 2 (Prior Art) is a block diagram of a conventional synchronous memory array. The memory array includes a memory cell 56 comprising one NMOS transistor N and one capacitor Cs. One terminal of the capacitor Cs is connected to the NMOS transistor N through a resistor Rc, and the other terminal is supplied with cell plate voltage VPLT. If an word line WL is selected according to an address signal and then sense amplifier enable signal SAEN is activated, a sense amplifier 53 amplifies data of the bit line BL and bit bar line /BL. Switch circuit units 51 and 52 switch the I/O lines IO, /IO and bit lines BL, /BL according to a column decoder signal Y_sw. An equalizing circuit 54 equalizes the bit lines BL, /BL according to an equalize signal EQ. A division circuit 55 divides the bit lines BL, /BL according to a control signal ISO.
FIG. 3 (Prior Art) is a timing diagram of an array operation of the conventional synchronous memory device. The operation of the conventional synchronous memory device is controlled according to a command signal CMD synchronous with an external clock signal CLK. In the command signal CMD, ACT means active command, WT means write command, RD means read command, and PRE means precharge command. In FIG. 3, SAEN is a sense amplifier enable signal, BL and /BL are bit line signals, CELL is a voltage signal of a memory cell, Y_sw is a column decoder signal. And in FIG. 3, VDD indicates power supply voltage, VPP indicates high voltage, and VSS indicates substrate voltage.
Firstly, the memory array is controlled in an active state according to the active command ACT (ACTIVE_STATE=high), a specific word line is selected according to the row address (Word Line=VDDxcx9cVPP), and thus a data of the memory cell is applied to the bit line BL and the bit bar line /BL according to charge sharing. Here, the bit line BL and the bit bar line /BL have a small potential difference due to a capacitance ratio between the memory cell and the bit line, which is sensed to a wanted potential by the operation of the sense amplifier (SAEN=high).
Thereafter, when a column command signal R or W is externally inputted, data can be inputted to the memory device and outputted from the memory device through a date in/out pin(not shown in the figures) according to the operation of the data IN/OUT buffer 24, the sense amplifier unit 46 and the I/O unit 48. At this time, if a succeeding column command is the write command W and the inputted data is different from the data stored in the memory array, the bit line BL, the bit bar line /BL and the memory cell renew data of the memory cell according to the operation indicated by a dotted line of FIG. 3. The data of the memory cell are renewed later than the data of the bit line BL and the bit bar line /BL. Such a delay is generated by a parasitic resistance element such as a transistor and a contact hole of the memory cell due to integration of the memory device.
After the column operation of the write and read operation, a data storage state of the memory cell is normally maintained and the precharge control signal PRE is applied to perform a succeeding row access operation. When the precharge control signal PRE is inputted, the data of the memory cell are maintained, the word line, the sense amplifier enable signal SAEN, the bit line BL and the bit bar line /BL are sequentially disabled, and the memory device is prepared to perform the succeeding row access operation. As described above, the operation of the conventional synchronous memory device is performed according to the command CMD synchronous with clock signal CLK.
FIG. 4 (Prior Art) is a timing diagram of the write operation of the conventional synchronous memory device. When the active command signal ACT synchronous with a rising period of clock signal CLK is inputted, the memory array is controlled in an active state (ACTIVE_STATE=high). Thereafter, when a write command signal WT is inputted in the active state of the memory array (ACTIVE_STATE=high), a series of operations are performed to input a write data through a DQ pin and store the data in the memory array. When the precharge control signal PRE is inputted, the active state signal ACTIVE_state of the memory array enabled in a high level is disabled in a low level. Accordingly, in the write operation of the conventional synchronous memory device, the active state signal ACTIVE_state of the memory array is set to a high level according to the active command signal ACT, and reset to a low level according to the precharge control signal PRE.
FIG. 5 (Prior Art) is a circuit diagram illustrating the precharge control signal generator of the conventional synchronous memory device. The precharge control signal generator 60 includes a controller 362 receiving the write signal W from the command decoder 26, the test signal TEST from the test mode detector 34 and the write enable signal /WEI from the command buffer 20, and controls a test mode state. An RS flip flop unit 364 latches the output signal from the controller 362. A pulse generator 366 detects a falling edge of the output signal FFOUT from the RS flip flop unit 364, and generates a short pulse and the test mode precharge control signal TPRE. OR gate 38 ORs the test mode precharge control signal TPRE from the pulse generator 366 and the precharge control signal PRE of a normal mode from the command decoder 26, and generates the precharge control signal PRE.
The controller 362 includes an inverter 368 receiving the write signal W from the command decoder 26, and outputting an inverted signal. An inverter 367 receives the test signal TEST from the test mode detector 34, and outputs an inverted signal. An OR gate 369 ORs the output signals from the inverters 367, 368. A NOR gate 370 NORs the output signal from the inverter 367 and the write enable signal /WEI from the command buffer 20.
The RS flip flop unit 364 includes a NAND gate 371 receiving the output signal from the OR gate 369 as a reset signal. A NAND gate 372 receives the output signal from the NOR gate 370 as a set signal. In addition, the output terminal of the NAND gate 371 is connected to the input terminal of the NAND gate 372, and the output terminal of the NAND gate 372 is connected to the output terminal of the NAND gate 371.
The pulse generator 366 includes a delay 373 delaying the output signal FFOUT from the RS flip flop unit 364 for a predetermined time. An inverter 374 receives the output signal from the delay 373. A NOR gate 375 NORs the output signal FFOUT from the RS flip flop unit 364 and the output signal from the inverter 374, and generates the test mode precharge control signal TPRE.
The operation of the conventional precharge control signal generator 60 will now be explained with reference to the operation timing diagram of FIG. 6 (Prior Art). When the output signal from the test mode detector 34 is maintained at a high level according to the test mode setting, if the write command WT is inputted, the write signal W generates a high level short pulse at the rising edge of the clock signal CLK. The write signal W resets the RS flip flop unit 364 to maintain its output signal FFOUT in a high level. Thereafter, when the write enable bar signal /WEI is transited to a high level, the output signal FFOUT from the RS flip flop unit 364 is transited to a low level. In addition, the test mode precharge control signal TPRE becomes a high level short pulse by the pulse generator 366, and the precharge control signal PRE from the command decoder 26 also becomes a high level short pulse.
Therefore, the conventional precharge control signal generator 60 generates a precharge signal synchronous with the rising edge of the clock signal CLK according to the precharge control signal PRE, and a precharge signal asynchronous from the clock signal CLK according to the test mode precharge control signal TPRE.
However, the conventional synchronous memory device has a disadvantage in that the precharge control signal generator can generate the precharge control signal for the test mode after input of the write command, when at least a hold time tWR of the write command elapses (refer to FIG. 6).
Accordingly, when the conventional synchronous memory device generating the precharge control signal according to the command signal intends to perform the test operation by using a tester having a lower frequency than its operation speed, to shorten an interval between the commands more than a period of the clock signal for the test operation, or to perform an inverse operation thereon, the memory device cannot adjust the interval between the commands due to the hold time of the command, and thus is not able to perform the test operation.
Moreover, the conventional synchronous memory device employs active circuits and write/read circuits by commands, thus increasing a layout area.
The inventions claimed and described herein provide a synchronous memory device having an asynchronous precharge function which can perform a test operation regardless of performance of a memory tester system.
They also provide a synchronous memory device having an asynchronous precharge function which can input an interval between commands regardless of a clock period, by generating a precharge control signal asynchronous from an externally-inputted clock signal by using a clock enable signal or chip selector bar signal which is not synchronous with the clock signal, when intending to generate the precharge control signal for a test mode after input of a read or write command.
Furthermore, the inventions claimed and described herein provide a synchronous memory device having an asynchronous precharge function which allows for a reduced layout area by removing circuits that would otherwise be required for each command, by generating a precharge control signal asynchronous from a clock signal.
The inventions provide a semiconductor memory device with a normal mode and a test mode. The memory device includes a memory cell array for storing data; and a precharge control signal generator for generating a precharge control signal in the test mode, by using a predetermined control signal which does not influence access to the data stored in the memory cell array, even when maintained in a high or low level in the test mode.
The precharge control signal generator receives the predetermined control signal, outputs a signal having an identical state to the control signal in the normal mode, and outputs a signal fixed in a high or low level in the test mode. Preferably, the precharge control signal generator includes a delay unit for equalizing a setting time of the precharge control signal after input of a command in the normal mode to a setting time of the precharge control signal after input of the control signal in the test mode. When input of the commands is synchronous with a rising edge of the clock signal in the normal mode, the precharge control signal generator generates the precharge control signal synchronously with the rising edge of the control signal in the test mode.
In addition, a synchronous memory device having an asynchronous precharge function includes a memory cell array unit having a plurality of memory cells for storing a plurality of data. A CLK/CKE buffer receives an external clock signal and a clock enable signal, and generates an internal clock signal and an internal clock enable signal. A command buffer receives external commands such as a chip selector bar signal, a ras bar signal, a cas bar signal and a write bar signal. An address buffer receives external addresses. A data I/O buffer inputs/outputs data signals. A command and state unit receives the address signal from the address buffer, the internal clock signal from the CLK/CKE buffer, the output signal from the command buffer and the internal clock enable signal from the precharge control signal generator, and generates control signals for controlling internal operations. A mode register receives the address signal from the address buffer and the mode selection signal from the command and state unit, and outputs control signals for controlling operation modes. A row/column address control and decoding circuit accesses a predetermined position of the memory cell array unit according to the address signal from the address buffer, and controls a read/write operation. An I/O data processing circuit controls data input/output in the read/write operation according to the output signal from the command and state unit. An asynchronous precharge control signal generator receives the internal precharge control signal from the command and state unit, the internal clock enable signal from the CLK/CKE buffer and the test mode signal from the mode register, and generates the internal clock enable signal and the precharge control signal.
The row/column address control and decoding circuit includes a row latch unit receiving the address signal from the address buffer, and latching a row address signal. A row predecoder predecodes the row address signal from the row latch unit. A row decoder decodes the output signal from the row predecoder, and selects a word line of the memory cell array unit. A column latch unit receives the column address signal from the address buffer, and latches a column address signal. A column predecoder predecodes the column address signal from the column latch unit. A column decoder decodes the output signal from the column predecoder and selects a bit line of the memory cell array unit.
The I/O data processing circuit includes a sense amplifier unit sensing and amplifying data of the bit line, when data are written/read to/from the memory cell array unit. A read control unit controls the read data from the sense amplifier unit to be transmitted to the data I/O buffer. A write control unit controls the write data from the data I/O buffer to be transmitted to the sense amplifier unit.
The precharge control signal generator includes a delay chain delaying the internal clock enable signal from the CLK/CKE buffer for a predetermined time. An AND gate ANDs the test mode detection signal from the mode register for controlling a precharge mode and the output signal from the delay chain. An OR gate ORs the internal precharge control signal from the command and state unit and the output signal from the AND gate, and outputs the precharge control signal. An OR gate ORs the test mode detection signal and the internal clock enable signal, and generates the internal clock enable signal.
The delay chain includes an even number of inverters.
According to another aspect of the present invention, a synchronous memory device includes a memory cell array unit having a plurality of memory cells for storing a plurality of data. A CLK/CKE buffer receives an external clock signal and a clock enable signal, and generates an internal clock signal. A command buffer receives external commands such as a chip selector bar signal, a ras bar signal, a cas bar signal and a write bar signal. An address buffer receiving external addresses. A data I/O buffer inputs/outputs data signals. A command and state unit receives the address signal from the address buffer, the internal clock signal from the CLK/CKE buffer, the output signal from the command buffer and the internal chip selector bar signal from the precharge control signal generator, and generates control signals for controlling internal operations. A mode register receives the address signal from the address buffer and the mode selection signal from the command and state unit, and outputs control signals for controlling operation modes. A row/column address control and decoding circuit accesses a predetermined position of the memory cell array unit according to the address signal from the address buffer, and controls a read/write operation. An I/O data processing circuit controls data input/output in the read/write operation according to the output signal from the command and state unit. An asynchronous precharge control signal generator receives the internal precharge control signal from the command and state unit, the internal chip selector bar signal from the command buffer and the test mode signal from the mode register, and generates the internal chip selector bar signal and the precharge control signal.
The row/column address control and decoding circuit includes: a row latch unit receiving the address signal from the address buffer, and latching a row address signal. A row predecoder predecodes the row address signal from the row latch unit. A row decoder decodes the output signal from the row predecoder, and selects a word line of the memory cell array unit. A column latch unit receives the column address signal from the address buffer, and latches a column address signal. A column predecoder predecodes the column address signal from the column latch unit. A column decoder decodes the output signal from the column predecoder, and selects a bit line of the memory cell array unit.
The I/O data processing circuit includes a sense amplifier unit which senses and amplifies data of the bit line, when data are written/read to/from the memory cell array unit. A read control unit controlsg the read data from the sense amplifier unit to be transmitted to the data I/O buffer. A write control unit controls the write data from the data I/O buffer to be transmitted to the sense amplifier unit.
The precharge control signal generator includes a delay chain for delaying the internal chip selector bar signal from the command buffer for a predetermined time. An AND gate ANDs the test mode detection signal from the mode register for controlling a precharge mode and the output signal from the delay chain. An OR gate ORs the internal precharge control signal from the command and state unit and the output signal from the AND gate and outputs the precharge control signal. An AND gate ANDs an inverted signal of the test mode detection signal and the internal chip selector signal, and generates the internal chip selector bar signal.
The delay chain includes an even number of inverters.
According to still another aspect of the present invention, a synchronous memory device receiving a command for a read/write operation synchronously with an external clock signal, and having a memory cell storing a plurality of data and an I/O circuit inputting/outputting the data of the memory cell in the read/write operation, includes an asynchronous command input control signal generator for generating an asynchronous command input control signal asynchronous from the clock signal according to an input pulse signal asynchronous from the clock signal, and controlling input of an external command signal asynchronous from the clock signal according to the asynchronous command input control signal.
The input pulse signal asynchronous from the clock signal may be a clock enable signal. And the input pulse signal asynchronous from the clock signal may be a chip selector bar signal.
As discussed earlier, the synchronous memory device having the asynchronous precharge function can input an interval between the commands regardless of the clock period, by generating the precharge control signal PRE by using the clock enable signal CLK or chip selector bar signal /CS which is not synchronous with the clock signal CLK, when intending to generate the precharge control signal PRE for the test mode after input of the write command in the write operation.
Especially, in order to efficiently test a specific operation parameter of the high frequency synchronous memory device in a wafer or package level and screen a defect thereof, the test operation should be performed with a sufficient margin. The conventional synchronous DRAM using the precharge control signal generator cannot obtain a sufficient margin due to poor performance of the memory tester. However, the synchronous memory device having the asynchronous precharge function in accordance with the present invention can freely perform the test operation, regardless of performance of the memory tester.
That is, the synchronous memory device of the present invention can perform the sufficient margin test operation in the wafer level, and thus replaces a defect memory cell by a redundancy memory cell in the wafer level, which results in an improved yield. Moreover, the sufficient margin test can also be carried out in the package test, to improve quality.